The invention relates to fiber-reinforced ceramic composites. In particular, the invention relates to clutch disks for motor vehicles and a process for producing friction disks from these composites, where these friction disks comprise the fiber-reinforced ceramic composites, in particular those which are reinforced with carbon fibers, especially woven fabric-reinforced C/SiC core material (carbon fiber-reinforced materials whose matrix comprises silicon carbide) and a ceramic friction layer comprising predominantly SiC phases.
The use of mineral fibers for friction linings of clutches has been known for a long time. Asbestos materials are particularly suitable for this purpose. However, the use of these asbestos-containing materials is nowadays no longer acceptable from an ecological point of view, so that use is made virtually exclusively of materials based on other mineral fibers. Owing to the increased power of engines, vehicle speeds and weights of vehicles, the demands made of the tribological properties of clutch disks in the motor vehicle sector have also increased; for this reason, new material pairings are also being examined to an increasing extent.
In this context, carbon fiber-reinforced composites or ceramics, in particular those having an SiC-containing matrix, are of particular interest. These materials have a high thermal stability, low wear and good tribological properties.
In DE-A 199 39 545, clutch disks are described having friction linings made of carbon fiber-reinforced silicon carbide (SiC) to increase the stressability and performance compared to the prior art. A disadvantage here is that production of a dense SiC matrix reinforced with carbon fibers can be achieved only with difficulty on an industrial scale. A technical achievement in material synthesis has been achieved by infiltration of carbon fiber-reinforced carbon-containing intermediate bodies with liquid silicon, generally giving carbon fiber-reinforced composites having an Si and SiC matrix.
In WO-A 00/41982 ceramic bodies are proposed for brake disks which are composed of a core layer and a friction layer, with the core body being made up of at least one long fiber-reinforced layer, preferably with UD layers (layers of unidirectional tapes, i.e. tapes which comprise continuous parallel fibers) arranged at an angle to one another. Between the UD layers, there may be further layers of short fiber-reinforced material or woven fabric-reinforced material. In the case of high rotational speeds as are required, in particular, for clutch disks, it has been found that the UD fiber layers are relatively unsuitable because of the low tensile strength perpendicular to the fiber direction.
Short fiber-reinforced C/SiC variants have proven themselves in respect of oxidation resistance at very high temperatures and during long-term operation, particularly in the case of brake disks. However, the high rates of rotation required for clutch disks cannot be achieved with the short fiber-reinforced C/SiC variants.
In EP-A 1 124 074 and EP-A 1 124 071, brake disks are disclosed composed of C/SiC and production processes for these in which base material reinforced with short fiber bundles is additionally provided with long fibers for reinforcement. The long fibers extend around the contour of the brake disk. This is said to inhibit crack growth and increase the strength of the disk. However, the disks described do not have dedicated SiC-rich friction layers which have been found to be advantageous, in particular for brake and clutch disks which rotate at high speeds.
The production processes from EP-A 1 124 074 and EP-A 1 124 071 are relatively unsuitable for achieving high long fiber contents and a uniform long fiber distribution over the entire component. However, both are prerequisites for a further increase in the strength under rotational stress.
It is therefore an object of the invention to achieve an improvement compared to the prior art in the strength of fiber-reinforced friction bodies, in particular clutch disks, at high rates of rotation or rotational speeds. At the same time, good tribological properties of the friction surfaces and an advantageous oxidation behavior are to be achieved. The clutch disks should be able to be produced by the technically and economically efficient liquid silicization process.
This object is achieved by the provision of a fiber-reinforced ceramic composite which is reinforced by a plurality of layers of a multidirectional woven fiber fabric, with the individual layers being penetrated by regions of a matrix comprising phases of silicon carbide, metallic silicon and optionally carbon. The clutch disks or friction disks produced from this composite additionally have a composite ceramic friction layer having a mass fraction of SiC in the range from 30 to 99%.
The invention accordingly provides a fiber-reinforced ceramic composite which comprises at least two layers of a multidirectional woven fiber fabric as reinforcement, with at least 5% of the area of each layer of woven fiber fabric, preferably at least 10% and particularly preferably at least 15%, being permeated by matrix material. The matrix material can therefore also form a continuous phase in a direction perpendicular to the plane of the layers of woven fabric.
Here and in the following, the term xe2x80x9clayer of woven fiber fabricxe2x80x9d or xe2x80x9cfiber matxe2x80x9d refers to any flat structure made up of fibers, i.e. the terms include braids, knitted fabrics, and nonwovens.